1. Field of the Invention
The present invention relates to flexible plastic lids for plastic containers and more specifically to such lids which apply a seal to the external upper rim surface of associated bowl containers.
2. The Prior Art
Plastic storage containers and lids are well accepted commercial items. Such products have widespread use, particularly in the storage of foodstuffs. Typical containers are cylindrical bowls which are offered in various volumetric sizes. The lid which is affixed to each bowl, and which seals the container, is generally comprised of a top wall which merges into a downturned rim flange which, in turn, merges into a downturned skirt. The lid inner diameter is slightly smaller than the outside diameter of the container upper rim such that, when the lid is pressed downward over the container rim, the container rim flange first flexes outwardly over the container rim and then is resiliently drawn inward, whereby clasping the container outer surface in a sealing grip.
U.S. Pat. No. 4,471,880 teaches such a configuration. The lid is applied by a center applied pressure, flexes outward at the rim flange, and is drawn inward against the container outer surface to effectuate a hermetic seal. This container and lid has met with considerable commercial success and is effective in achieving satisfactory sealing of the container contents from ambient air.
However, certain shortcomings of the patented container, and in particular the lid, prevent it from representing an ideal solution to the consumer's needs. First, because of the effectiveness of the seal, the lid is difficult to remove from the container. Secondly, a horizontal terminal tab on the downturned skirt of the lid, which is intended to provide a grip for the application of an upward force to break the seal and remove the lid, is generally too small to afford the user a convenient grasp.
The first aforementioned deficiency is because the application of a vertical force on a horizontal flange is an ineffective way to break a seal which is maintained by forces operating in a horizontal direction. That is, the application of a vertical force on a horizontal flange primarily acts to bend the flange upward, and transfers little force in the horizontal direction by which to break the seal between the inner surface of the lid skirt and the outer surface of the container. In order to remove the lid of the prior art, one must essentially slide a portion of the lid upward by the application of a vertical force to the horizontal flange. However, the horizontally directed sealing forces between the lid and the container create substantial frictional resistance countering such vertical movement. Consequently, a substantial amount of force must be applied to effectuate separation of the lid by sliding it upward.
In fact, the amount of force required can exceed the capability of children and adults, and particularly elderly adults. Other containers are available in the prior art and commercial markets which attempt to make the lid easier to remove by making the horizontal flange larger. Making the flanges or tabs larger, however, while making them easier to grasp, does not make the lid easier to remove, for the problem caused by the inappropriate directional application of force to the tabs and flanges remains.